In recent years, lithium secondary batteries have been widely used as a power source for small-sized electronic apparatuses. A lithium secondary battery is mainly composed of a positive electrode, a nonaqueous electrolytic solution and a negative electrode. In particular, a lithium secondary battery composed of a lithium compound oxide such as LiCoO2 as a positive electrode and a carbon material or a lithium metal as a negative electrode is suitably employed. As a nonaqueous electrolytic solution of such a lithium secondary battery, a carbonate such as ethylene carbonate (EC) or propylene carbonate (PC) is suitably used.
There is, however, a demand for secondary batteries having further improved battery characteristics such as cyclic property and electric capacity.
A lithium secondary battery using, for example, LiCoO2, LiMn2O4 or LiNiO2 as a positive electrode brings about a reduction of its battery performance, when a part of the solvent of the nonaqueous electrolytic solution locally undergoes an oxidative decomposition during the charging, because the decomposed product hinders the desired electrochemical reaction. Such a reduction is considered to be attributed to an electrochemical oxidation of the solvent in the interface between the positive electrode material and the nonaqueous electrolytic solution.
Also, a lithium secondary battery using, for example, a highly crystalline carbon material, such as natural graphite or artificial graphite, as a negative electrode brings about a reduction of its battery performance, when the solvent of the nonaqueous electrolytic solution undergoes a reductive decomposition at the surface of the negative electrode during the charging. Even in the case of EC which is generally used as a solvent for the nonaqueous electrolytic solution, a part thereof undergoes a reductive decomposition during repeated charging and discharging.
For the purpose of improving the battery characteristics of such lithium secondary batteries, Japanese Unexamined Patent Publication JP-A-H08-96849 discloses use of an aliphatic saturated dicarboxylic ester such as dipropyl adipate and that the content of the ester in the solvent is preferably at least 5% by volume. In Example 1, the Japanese publication discloses a battery using 10% by volume of dimethyl adipate. The cycle property of the battery is, however, about 65% after 15 cycles and is not satisfactory.
Japanese Unexamined Patent Publication JP-A-H08-321311 discloses use of various organic compounds such as ketones, alcohols, aldehydes, esters, organic acids and organic silicon compounds in an amount of 1 to 20% by volume in a primary battery for the purpose of improving the storage characteristics thereof. In particular, the Japanese publication proposes an attempt to improve the storage characteristics of the primary battery, such as the self discharging rate, by the addition of a dialkyl oxalate such as dimethyl oxalate- or diethyl oxalate as the above organic compound. No consideration of a secondary battery is, however, made in the Japanese publication. The constitution of the electrode materials and the nonaqueous electrolytic solution used in the Japanese publication differs from those of a secondary battery.
Japanese Unexamined Patent Publication JP-A-H09-199172 discloses use of at least 1% by volume, preferably 20 to 80% by volume, of a dialkyl oxalate containing an alkyl group having 1 to 3 carbon atoms. Although the cycle property under conditions of a high voltage and a high load discharging is improved, the retention of the discharge capacity at the 100 cycles is only 82 to 92% of the discharge capacity at the 10 cycles.
Japanese Unexamined Patent Publication JP-A-H08-45545 discloses a lithium battery which includes a negative electrode containing a carbon material having a crystallization degree of greater than 0.8, and an electrolytic solution containing a lithium salt and an organic solvent mixture composed of the first solvent such as propylene carbonate, the second solvent such as dimethyl carbonate or diethyl carbonate and vinylene carbonate (VC) and which suppresses delamination of the carbon material. The cycle property of the battery, however, is not satisfactory.
Japanese Unexamined Patent Publication JP-A-2002-367674 discloses an electrolytic solution containing a lithium salt dissolved in a solvent composed mainly of at least one nonaqueous solvent selected from the group consisting of carbonic esters, ethers and lactones, and the electrolytic solution being characterized in that the solvent further contains a diester of a dicarboxylic acid and an aromatic compound such as dibenzofuran. No substantial consideration is made in the Japanese publication with respect to cycle property.
Japanese Unexamined Patent Publication JP-A-2000-3724 discloses a lithium secondary battery which is characterized in that the positive electrode is made of a material containing a lithium compound oxide, in that the negative electrode is made of a material containing graphite, in that the nonaqueous solvent contains as main ingredients a cyclic carbonate and a linear carbonate and in that the nonaqueous solvent further contains not less than 0.1% by weight but not more than 4% by weight of 1,3-propanesultone and/or 1,4-butanesultone. However the retention of the discharge capacity after 50 cycles at room temperature is 82 to 95%. Thus, an improvement of the long-term cycle property is demanded.
It is the object of the present invention to provide a lithium secondary battery which is excellent in long-term cycle property and in battery characteristics, such as electric capacity and storage property in charged condition, and to provide a nonaqueous electrolytic solution usable for such a lithium secondary battery to solve the problems of the above described nonaqueous electrolytic solution for lithium secondary batteries.